The present invention relates to inorganic fibers.
State of the art ceramic fibers from the system Al.sub.2 O.sub.3 -SiO.sub.2 with 40-75% by weight of Al.sub.2 O.sub.3 and no more than 2% by weight of alkaline- and alkaline earth oxide are X-ray amorphous, and prior art fibers with 80-95% by weight of Al.sub.2 O.sub.3, the rest being SiO.sub.2, are crystalline. Such fibers are used for thermal insulation at raised temperatures as mats and fibrous moldings, and for the manufacture of reinforced ceramic or metallic moldings.
The prior art ceramic fibers have a high resistance to water, i.e. they do not dissolve in water or in physiological solutions, or they dissolve only after a very long period of time. When heat is applied, the fibers crystallize out; or in the case of fibers that are already crystalline, such fibers show additional crystal growth. The form factor of the fiber, characterized by the length to diameter ratio, remains preserved in all cases. In addition, the X-ray amorphous fibers form, on account of their high SiO.sub.2 content, upon the application of heat, cristobalite, which causes silicosis.
Furthermore, there exists the idea that ceramic fibers can be cancer-causing, for which their resistance to dissolving in water or in body fluids is regarded as an important reason. They remain preserved in the body as fine fibrous foreign bodies similar to asbestos fibers, and could lead to irritations. Values for the so-called "network dissolution rate" and the residence time of different fibers in liquid are known from "Keramik und Glas", Reports of the Professional Association of the Ceramic and Glass Industry, March 1992, No. 1, pages 30-41, in particular pages 38-40.
According to EP 0 459 897 Al, a modified fiber, namely fibers based substantially on SiO.sub.2, Al.sub.2 O.sub.3 and alkaline earth oxides, which is supposed to decompose upon contact with a physiological environment, is obtained through the addition of P.sub.2 O.sub.5. The fibers have a significant SiO.sub.2 content and can be added at about 700.degree. C.